1. Field of the Invention
The present invention relates to a method and apparatus for dithering in an image-displaying device, where the dithering may provide representation of whole gray scales without saturation.
2. Description of the Related Art
Image-displaying devices for example, CRT display, TFT-LCD, and printers, have previously been developed. Displaying images may be divided into processes, which may include digitization of a real image for performing image processing, and displaying the processed signal through an image-display device. An image-displaying device may provide an image similar to a corresponding real image based on a series of processes. Data loss may be reduced during the digitization process of a real image, and the data loss which may occur due to the processing of the image, may also be reduced. The digitization process of a real image may include a series of sampling processes, for example quantization, and/or standardization, and may include signal transactions, which may occur in the digitization process. One goal of the digitization process may be to process a digital image as close as possible to the corresponding real image, while reducing data loss that may occur.
An image-displaying device may be used to display a processed image to the viewing characteristics of a corresponding user, and may be limited by this display requirement. An image-displaying device may be limited in the area of displaying gray scales. An example of displaying gray scales may be recognized when R (Red), G (Green), and B (Blue) 8-bit input data, and 8-bit gray scales are converted to a smaller bit scale. For example, input data may be represented by 28 gray scales, thus color combinations of R, G and B, 28×28×28 may result in 224 colors. When an image-display device converts 8-bit data to 6-bit data, a corresponding gray scale conversion 28−26 may not include each input data, similarly all of the original colors may not be expressed. Therefore, when an image-displaying device processes a signal by reducing gray scales to a level below the full gray scales of the corresponding original video signal, inputting noise to the image data (or ‘dithering’) may be used to provide an original image restoration process.
Each pixel may include three sub-pixels, for example R, G and B, and/or input data may be applied to each of these sub-pixels. If input data applied to the sub-pixels includes a reduction in the number of gray scales then a false contour line representing a definite contour line shown at the boundaries of an image, or Mach's phenomenon representing a bright band or dark band shown at the surface of screen may occur.
The false contour line or Mach's phenomenon may generate a contour line that does not exist in the real image, thus lowering the quality of an image displayed. Therefore, dithering, which may include inputting noise to pixels at the boundaries of images, may be used to avoid this type of false contour line. When for example, a bit width at a video source is larger than the bit width at an image-display device, then dithering may provide an increase in quality for that image-display device.
A type of dithering known as ‘truncation’ may be used to improve the quality of an image-display device. Truncation may include removing bits from a set of input data by removing the LSB (Least Significant Bit(s)), for example, 2 bits may be removed from an 8-bit signal, and the remaining 6-bit signal may be output to a pixel. When the remaining 6-bit signal is output to the pixel, the gray scales of one sub-pixel may be equal to 26 for showing the boundaries of an image.
FIG. ‘1’ illustrates an example of a truth table according to a conventional truncation method. Referring to FIG. 1, when input data provided by an 8-bit signal is represented with 6 bits, decimal values 0, 1, 2, and 3 for example may be converted to ‘0’, and decimal values of 4, 5, 6 and 7 may be converted to ‘1’. These converted values may be outputted to display an image on a screen, which may have a false contour line non-existent in the real image.
Another type of dithering is referred to as temporal/spatial or ‘temporal’ compensation. 8-bit input data may be converted to 6-bit output data by temporal compensation. Temporal compensation may provide the removal of 2 bits, 2 LSBs for example, from the 8-bit input data. Temporal compensation may be performed on each frame of output data received based on the 2 LSBs removed. In spatial compensation, each output data frame may be compensated based on the 2 LSBs removed, by considering the line and pixel locations in each output data frame. Therefore 8-bit output data that has been converted to 6-bit data, may be approximated by using 6-bit temporal/spatial compensation on the output data. The lines and pixels in each output data frame may be compensated by a weight which corresponds to the 2 LSBs of the 8-bit input data.
Table ‘1’ is an example of temporal/spatial compensation based on 2 LSBs.
TABLE 1A secondA fourthLSB 2 bitA first frameframeA third frameframe00————01———+110+1—+1—11+1+1—+1
Referring to Table 1, the (MSB) 6-bit data may be outputted without adding a weight, or may be summed with a weight of 1. The 6-bit data may be outputted to one pixel based on 2 LSBs of data from respective output data frames. For example, when 2 LSBs of data are removed from 8-bit data during a time period corresponding to four frames of data, and the data is equal to ‘11’, then the output data may lose 3 sets of LSBs ‘11’×4 frames resulting in a value of 12 . The lost value may be compensated by:
adding +1(‘100’) to the 6 MSBs of a corresponding pixel in a first frame,
adding +1 to the 6 MSBs of the corresponding pixel in a second frame,
outputting 6 MSBs of the corresponding pixel in a third frame without change, and                adding +1 to the 6 MSBs of the corresponding pixel in a fourth frame.        
The compensation may further include four frames of (‘100’)×3 (3 being the number of frames to which ‘1’ is added), i.e. a value of 12 may be compensated so that the value lost during a time period corresponding to four frames and the compensated value are the same.
When the 2 LSBs are for example ‘10’, the lost value during a time period corresponding to four frames may be represented by 2 LSBs ‘10’×four frames, resulting in 8 bits.
The lost value may be compensated by:
adding +1(‘100’) to 6 MSBs of a corresponding pixel in a first frame,
outputting 6 MSBs of the corresponding pixel in a second frame without change,
adding +1 to 6 MSBs of the corresponding pixel in a third frame, and
outputting 6 MSBs of the corresponding pixel in a fourth frame without change.
Temporal/spatial compensation may not be limited to selecting a frame and adding a weight to the frame selected. For example, when the 2 LSBs are ‘11’ a weight may be added to a pixel corresponding to three frames among four continuous frames, and when the 2 LSBs are ‘10’, a weight may be added to a pixel corresponding to the two frames among four continuous frames.
FIG. 2 is an exemplary truth table of a conventional temporal/spatial compensation for 8-bit input data.
Referring to FIG. 2, 8-bit input data having corresponding gray scale values equal to or greater than ‘252’ may not change.
FIG. 3 is a graph illustrating exemplary output characteristics according to a conventional temporal/spatial compensation.
Referring to FIG. 3, the output data may be saturated to a specific value irrespective of a change to the input data illustrated in FIG. 2.
Therefore, a conventional dithering process based on conventional temporal/spatial compensation, may not provide an expression of input data for representing a high level of brightness.